Oxygen atom transfer from (NH3)(4)(H2O)RhOOH2+ to organic and inorganic nucleophiles takes place according to the rate law -d[(NH3)(4)(H2O)RhOOH2+]/dt = k[H+] [(NH3)(4)(H2O)RhOOH2+][nucleophile] for all the cases examined. The third-order rate constants were determined in aqueous solutions at 25 degreesC for (CH2)(5)S (k = 430 M-2 s(-1), mu = 0.10 M), (CH2)(4)S-2 (182, mu = 0.10 M), CH3CH2SH (8.0, mu = 0.20 M), (en)(2)Co(SCH2CH2NH2)(2+) (711, mu = 0.20 M), and, in acetonitrile-water, CH3SPh (130, 10% AN, mu = 0.20 M), PPh3 (3.74 x 10(3), 50% AN), and (2-C3H7)(2)S (45, 50% AN, mu = 0.20 M). Oxidation of PPh3 by (NH3)(4)(H2O)(RhOOH2+)-O-18-O-18 produced (OPPh3)-O-18. The reaction with a series of p-substituted triphenylphosphines yielded a linear Hammett relationship with rho = -0.53. Nitrous acid (k = 891 M-2 s(-1)) is less reactive than the more nucleophilic nitrite ion (k = 1.54 x 10(4) M-2 s(-1)).